Arsenic (As) is a potent environmental carcinogen. Tens of millions of people worldwide suffer of diseases caused by consumption of water with high levels of iAs. Paradoxically, As-trioxide (ATO), a trivalent form of iAs, is widely used for treatment of acute promyelocytic leukemia (APL). Metabolism of iAs plays an important role in both the adverse effects of chronic exposures to iAs and the therapeutic effects of ATO. Liver is the primer site for metabolic conversions of iAs in humans. Our ongoing project provides evidence that As(+3 oxidation state) methyltransferase (AS3MT) is the key enzyme in the pathway for iAs methylation in human liver. Human hepatocytes that express AS3MT methylate iAs to methyl-As (MAs) and dimethyl-As (DMAs) species that contain either AsIII or AsV. These metabolites are also found in urine of individuals exposed to iAs from environmental sources and patients treated with ATO. Evidence from laboratory, clinical and population studies suggest that the AS3MT-catalyzed formation of methylated trivalent arsenicals, MAsIII or DMAsIII, is an important mechanism for activation of iAs as a toxin and carcinogen and that these metabolites may also play significant roles in the therapeutic efficacy of ATO or adverse side effects associated with ATO treatment. Thus, the overall toxic or carcinogenic effects related to iAs exposures or the outcomes of ATO therapy depend on the capacity of the liver to convert iAs species to MAs and DMAs and on the distribution, retention, and clearance of the biologically active trivalent metabolites, iAsIII, MAsIII and DMAsIII, in target cells and tissues. Results of our ongoing project show significant interindividual variations in the metabolism of iAs by cultured primary human hepatocytes which are consistent with variations in urinary profiles of iAs metabolites in populations exposed to iAs. AS3MT polymorphism has been shown to contribute to the metabolic variations in both population and cell culture studies. However, the rate of iAs methylation in human hepatocytes does not correlate with AS3MT expression, suggesting that in addition to AS3MT polymorphism, other factors play significant roles in determining the capacity of human hepatocytes to methylate iAs. The main goal of this competing continuation application is to further characterize the role of AS3MT polymorphs in the metabolism of iAs and to identify AS3MT co-factors and other enzymes or transport systems that play significant roles in the metabolism of iAs by human hepatocytes. The enzymes and co- factors that support AS3MT-catalyzed methylation of iAs or facilitate the metabolic conversions of iAs independently of AS3MT will be identified. Relationship between the structure, catalytic function, and co-factor requirements for the most common polymorphs of human AS3MT will be characterized. Finally, membrane transporters that regulate the uptake of iAs and reuptake or efflux of the methylated metabolites of iAs will be identified and their roles in the metabolism of iAs in human hepatocytes will be characterized. Results of this work will provide novel information about the enzymatic and transport systems that, taken together, determine the capacity of human liver to metabolize iAs and thus, play a critical role in modulation of both the adverse effects associated with environmental exposures to iAs and the outcomes of cancer therapies involving ATO. PUBLIC HEALTH RELEVANCE: Inorganic arsenic is a potent human carcinogen that contaminates drinking water supplies worldwide. However, arsenic is also used for treatment of leukemia. Chemical transformations of arsenic in the liver affect both the diseases associated with consumption of arsenic-contaminated water and the outcomes of leukemia therapy. This project will provide information about key processes that regulate the transformation of arsenic in human liver. Results of this work may help to protect people against toxicity of arsenic and to improve the efficiency of leukemia therapy. .